Reverberation artifact cancellation in ultrasonic diagnostic images

ABSTRACT

An ultrasonic diagnostic imaging system acquires received beams of echo signals produced in response to a plurality of transmit events. The received beams are combined with refocusing to account for differences in receive beam to transmit event locations. The delays and weights used in the refocusing are supplemented with delays and weights which correct for reverberation artifacts. The received echo signals are processed to detect the presence of reverberation artifacts and a simulated transmission of reverberation signal components to virtual point sources in the image field is calculated. This simulation produces the delays and weights used for reverberation signal compensation, or estimated reverberation signals which can be subtracted from received echo signals to reduce reverberation artifacts.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2018/062158, filed on May9, 2018, which claims the benefit of U.S. Provisional Patent ApplicationNo. 62/504,681, filed on May 11, 2017. These applications are herebyincorporated by reference herein.

TECHNICAL FIELD

This invention relates to medical diagnostic ultrasound systems and, inparticular, to ultrasound systems which reduce or cancel image artifactsdue to reverberation echoes in ultrasound images.

BACKGROUND

Medical ultrasound images can become contaminated with image artifactswhich originate from various sources. In addition to simple noiseartifacts, which can arise when an unsatisfactory signal to noise ratiois extant, other artifacts are particular to phenomena of ultrasound.Speckle artifacts arise by reason of the coherent nature of ultrasoundsignals. These artifacts, which can appear as a mild watermark in theimage, can be reduced by signal processing techniques such as frequencycompounding and spatial compounding. Aberration artifacts can arise dueto differences in the speed of sound through different tissues andsubstances in the ultrasound paths to and from the transducer and can bereduced by delay compensation in the beamforming process. Another imageartifact which is particular to ultrasound is reverberation artifact.Reverberation occurs when a transmitted ultrasound wave is reflectedback by a strong reflector in the near field of the image region andtravels back to the face of the transducer, which acts as a reflector tobounce the returning wave outward again, thereby introducing a secondoutward wave into the image field during echo reception. Thisreverberated wave will result in its own echo returns which willintermingle with echoes returning from the transmit wave. Although thereverberation echoes are at a lower level than those returning from thetransmit wave, they are nonetheless often of sufficient amplitude toproduce their own partial phantom image overlaid on the primary desiredimage.

U.S. Pat. No. 6,905,465 (Angelsen et al.) describes a technique forcorrecting for reverberation aberrations in ultrasound imaging bytransmitting twice, once to sample the returning signals forreverberation signal artifacts, then a second time in which thetransmission is adjusted to reduce the effects of reverberation.However, this approach requires two transmit events, which increases thetime required to acquire the image data and hence reduces the frame rateof display.

SUMMARY

The present invention aims to desirably reduce or eliminatereverberation artifacts from ultrasound images, without the need formultiple transmissions that reduce the display frame rate.

In accordance with the principles of the present invention, a diagnosticultrasound system and method are described which reduce the appearanceof reverberation artifacts in ultrasound images. The method and systemoperate by first detecting the presence and location of reverberationartifacts in ultrasound image data by operating on a set of echo signaldata. The signal components of the received echo signal data whichproduce the reverberation artifacts are estimated, preferably using theprinciples of retrospective dynamic transmit focusing. The estimatedreverberation signals are subtracted from the actual received signals,or offsetting phase and weight adjustment used in the beamformingprocess to reduce or eliminate the reverberation artifacts from theimage data used to produce an ultrasound image.

In the drawings:

FIGS. 1 a) through e) illustrate the phenomenon of reverberationartifacts in an ultrasound image.

FIG. 2 illustrates in block diagram form an ultrasound systemconstructed in accordance with the principles of the present invention.

FIG. 3 illustrates the retrospective dynamic transmit focusingprinciple.

FIGS. 4 a) through c) illustrate the simulation of a complex wavefrontdecomposed to virtual point sources in an estimation of reverberationartifact signals.

FIG. 5 illustrates the reverberation signal detection and processingperformed by the reverberation signal processor of FIG. 2.

The drawings of FIGS. 1 a) through e) illustrate the problem ofreverberation artifacts in ultrasound imaging. In FIG. 1 a) a transducerarray 104 is shown transmitting a plane wave of ultrasound energy 20toward a target anatomy 10. In the path between the transducer and thetarget are a number of specular reflectors 12 which will reflect some ofthe transmitted ultrasound energy back toward the transducer. Thesereflectors may be different tissues, bone such as ribs or the skull, aircavities, or other structures which present a significant acousticimpedance discontinuity. FIG. 1 b) shows echoes 22 being reflected backtoward the transducer from the reflectors 12. In FIG. 1 c) the echoes 22have been reflected off of the face of the transducer array 104 and asecondary (reverberation) energy wave 22′, mirroring that previouslyreturned by the reflectors 12, now propagates back out through the imagefield. Meanwhile, the original transmit wave has been reflected backfrom the target anatomy 10 and echoes therefrom are returning to thetransducer array as shown by wavefront 24. When the reverberation energy22′ reaches the reflectors 12 some of the energy is reflected backtoward the transducer a second time. These secondary reverberationechoes are shown returning to the transducer array at 26 in FIG. 1 d),where they are intermingled with the echoes returning from the targetanatomy. The image produced by these different echoes is shown on screen124 in FIG. 1 e. The sharp image of the specular reflectors 12 on theleft side of the image is constructed from reception of the first set ofechoes 22 returned by the reflectors. The image of the target region 10is constructed from reception of the echoes 24 returned by the targetanatomy. But the target anatomy is partially obscured by artifacts ofthe reverberation echoes returned a second time by reflectors 12 which,by reason of their time of travel that has them intermingling withreturning echoes from the target anatomy, produce a reconstructedphantom image which overlays the image of the target anatomy in thisexample. It is an object of the present invention to detectreverberation echoes and cancel their effects in the imagereconstruction, but to do so without using any additional“interrogating” transmissions by the array transducer, which would havethe undesirable effect of reducing the frame rate of display of theimage.

FIG. 2 illustrates in block diagram form an ultrasound imaging systemconstructed in accordance with the principles of the present invention.An ultrasound probe 102 includes a transducer array 104 of transducerelements. Selected groups of the transducer elements are actuated atrespectively phase delayed times by a transmit beamformer 106 totransmit beams steered and focused at selected focal regions in thedesired directions and from the desired origin(s) along the array. Thetransmit beamformer is coupled to the transducer elements by atransmit/receive switch which may comprise a crosspoint switch thatprotects the receiver inputs from the high voltage transmit pulsesapplied. The echoes received by each transducer element of the array 104in response to each transmit beam are applied to the inputs of multilineprocessors 110 a-110 n. Each multiline processor comprises a receivebeamformer which applies its own set of delays and, if desired,apodization weights to weight the received echoes from the arrayelements to form a differently steered and focused receive beam from thesame transmit beam. Suitable multiline beamformers for the multilineprocessors 110 a-110 n are described, for instance, in U.S. Pat. No.6,695,783 (Henderson et al.) and U.S. Pat. No. 5,318,033 (Savord). Thescanline outputs of the multiline processors 110 a-110 n are coupled toa line store 112 which stores the received multilines at least until allof the multilines needed to form a scanline of display data have beenacquired.

The received multilines are combined by a combiner 90, which performsprocessing the received signal prior to their scan conversion. Thecombiner may comprise several units such as multiplier 116, weightingcircuits 114, delay 118 and summer 120. The group of multilines used toform a particular line of display data are applied to respective ones ofmultipliers 116 a-116 n to produce the display data for thecorresponding scanline location. The echo data from each line may, ifdesired be weighted by apodization weights 114 a-114 n. In general,these weights will weight each line as a function of its round-tripimpulse response. A suitable weighting algorithm can be derived byletting the term amplitude(x,y) be the insonification amplitude of apoint at location (x,y) in the image field by the transmit wave-front,the azimuth position x=0 corresponding to the center axis of thetransmit beam. Let X be the azimuth of a received multiline with respectto the transmit beam axis. The weight applied to this received multilineto form a point of the image at depth Y is:Weight(X,Y)=amplitude(X,Y)For determination of an appropriate delay characteristic, letpropagation_time(x,y) be the propagation time needed by the transmitwavefront to reach a point at location (x,y), the azimuth x=0corresponding again to the center axis of the transmit beam. Let X bethe azimuth of the received line with respect to the transmit beam axis.The delay applied to this received multiline to form a point of theimage at depth Y is:Delay(X,Y)=propagation_time(X,Y)−propagation_time(0,Y)where propagation_time(0,Y) is the time to reach a point at the samedepth but on-axis.

The functions amplitude(X,Y) and propagation_time(X,Y) may, for example,be obtained from a simulation of the transmit field. An appropriate wayto compute the propagation time is to use the phase delay of the fieldfrom monochromatic simulation at several frequencies. The amplitude maybe computed by averaging the amplitude of the field at severalfrequencies. In addition, a depth-dependent normalization can be appliedto the weights. This multiplies all the weights at a given depth by acommon factor. For example, the normalization can be chosen so thatspeckle regions have uniform brightness with depth. By varying theweights as a function of depth, it is possible to vary the size andshape (apodization) of the aperture dynamically with depth.

The amplitude and propagation time do not need to be derived from asimulation of the exact transmit characteristics used in the system. Thedesigner may choose to use a different aperture size or a differentapodization for example.

The echoes from each line are weighted by the multipliers 116 a-116 nand delayed by delay lines 118 a-118 n. In general, these delays will berelated to the location of the transmit beam center to the receive linelocation as shown above. The delays are used to equalize the phase shiftvariance that exists from line to line for the multilines with differingtransmit-receive beam location combinations, so that signal cancellationwill not be caused by phase differences of the signals combined fromdifferent transmit apertures.

It will be appreciated that in a digital system the delay lines may beeffected by storing the weighted multiline echo data in memory andreading the data out at later times which effect the necessary delay.Shift registers of differing lengths and clock signals may also be usedto effect a digital delay, or an interpolating beamformer such as thatdescribed in the aforementioned U.S. Pat. No. 6,695,783 may be used. Thedelayed signals are combined by a summer 120 and the resultant signalsare coupled to an image processor 122. The image processor may performscan conversion or other processing to improve the displayed image. Theresultant image is displayed on an image display 124.

In the system of FIG. 5 the delay lines 118 and summer 120 effect arefocusing of the signals received from the several receive multilineswhich are co-aligned in a given direction. The refocusing adjusts forthe phase differences resulting from the use of different transmit beamlocations for each multiline, preventing undesired phase cancellation inthe combined signals. The weights 114 weight the contributions of themultilines in relation to the proximity of the transmit beam to themultiline location, giving higher weight to receive beams with highersignal-to-noise ratios. This results in an extended depth of field alongeach receive line and an enhanced penetration (improved signal-to-noiseratio) due to the combination of multiple samplings in each receive linedirection.

This refocusing of co-aligned received multilines also causes aretrospective dynamic transmit focusing effect as explained withreference to FIG. 3. In this drawing four transmit beams 34 aretransmitted by the transducer array 104, the transmit events beingreferred to as Tx1, Tx2, Tx3, and Tx4. Each transmit beam is transmittedin a different direction in the image field; in this example thetransmit and receive beams are in parallel, and so each transmit beamoriginates from a different location (x dimension) along the transducerarray. The energy of each transmission has a transmit beam profilegenerally indicated by the hourglass-shaped lines 30, which converges ata focal point indicated by arrow 32. Thus, each transmit beam isgenerated by its own transmit subaperture of the array. Following eachtransmission a receive line co-located with the transmit line isreceived and beamformed, and an additional receive beam 40 is receivedby the array transducer, each time at the same location (x direction)along the array. Two echo signal locations are marked by circles at thesame depth y on each receive beam 40, one at a shallow depth and theother at a deeper depth. The receive beam for the first transmit eventTx1 is shown at 42. It is seen that for the first transmit event thereceive beam 40 is offset to the right of the transmit beam 34 and sincedistance and time are equivalent in beamformation, the echoes receivedfrom the circled echo signal locations are located as shown on receivedbeam 42 following beamforming.

In the second transmit-receive cycle of Tx2 the transmit beam 34 islaterally closer to the receive beam 40. This smaller offset results inthe two circled echo signal locations being located closer to the focalpoint of the receive beam, as shown on the second receive beam 42′. Inthe third transmit-receive cycle of Tx3 the transmit beam is locatedlaterally to the right of receive beam 40, separated by the same lateraldistance as the Tx2 cycle. The echoes of the circled echo signallocation are located as shown by the signals on receive beam 42″. In thefourth transmit-receive cycle of Tx4 the transmit beam is locatedfurther to the right of the receive beam center, offset by the samedistance as in the case of the first Tx1 cycle. The echoes of thecircled echo signal location are located as shown by the signals onreceive beam 42′″, separated a greater distance from the receive beamfocal point. The variation in the circled echo signal locations may berepresented by curves 44 and 46 drawn across the four receive beams 42,42′, 42″ and 42′″. This variation is corrected in retrospective dynamictransmit refocusing by applying a delay correction to the respectivereceive multilines before combining them. An exemplary delay curveprofile is shown in U.S. Pat. No. 8,137,272 (Cooley et al.) forinstance. When this compensating delay is applied by means of delaylines 118 a-118 n in FIG. 2 the circled echo locations are allreproduced at the same respective depths along the multilines 42-42′″,as shown by circled depths 45 and 47, which are at corresponding receiveline depths as shown by straight curves 54 and 56. The four correctedmultilines may then be combined with weighting corresponding to thetransmit-to-receive line offset to produce the final display scanline 50with combined echo signals 52 and 58.

In accordance with the principles of the present invention theultrasound system of FIG. 2 further includes a reverberation signalprocessor 100. The purpose of the reverberation signal processor is toidentify the presence of reverberation signal artifacts in the echosignals received from the image field then, through a process of timereversal of the received reverb signal components back to virtual pointsources located at the transmit beam focal points, estimate the phasingand weighting which would be used to generate the reverb signals if infact they were to be transmitted and received. But no additionaltransmission and reception is employed. Instead, the estimated phasesand weights are used to supplement the phases and weights used in theretrospective dynamic transmit refocusing, thereby obtaining thecancellation signals for reverberation signal artifacts in the receivedecho signals used for imaging.

This process begins by detecting the presence of reverberation signalartifacts in the received echo signals. This is done by operating onenvelope-detected signals of the received multilines. In theimplementation of FIG. 2 the initially beamformed signals produced bythe multiline processors are envelope-detected by an envelope detector92. A strong reflector which is the source of reverberations can bedefined as any image pixel p(x,y) which is in the near field and closeto the transducer, such as a pixel within the first half of the image,that is, y≤y_(max)/2, where y indicates the image depth. The receivedsignal from this strong reflector should also have a value which isgreater than a threshold value of the intensity range, e.g., at least80% of the maximum pixel intensity. For instance, if the pixel valuerange is 0 to 255, an 80% threshold value would be 204, assuring thatany pixel suspected of containing the signal which causes the reverb hasa substantially large amplitude. The intensity value of the suspectedpixel should be at least two standard deviations greater than the meanintensity of a region of interest consisting of neighboring pixels, tomake sure that the overall increased image gain is not causing the highintensity. Once these bright, high intensity pixels are identified inthe image signals then beamformed r.f. lines (A-lines) corresponding tothese high intensity points are investigated to find the strong echoes.In this example the beamformed A-lines are the detected multilinesignals and pixel location p(x,y) corresponds to a signal at A_(i)(t)where A_(i) is the i'th A-line. For a linear array geometry the imagedepth y is directly related to the time (t_(e)) when the echo occurs,y=c₀t_(e)/2, where c₀ is the speed of sound in the imaging medium, andthe line number i is the closest ultrasound beam to the lateralx-position of p. This may be expressed as the integer value ofi=((x−x₁)/Δx)+1, where x₁ is the lateral position of the first A-lineand Δx is the distance between two consecutive A-lines. For a sectorscan geometry, in which the beams are angularly dispersed and spatiallyoriginate at a common apex location, the scan conversion needs to beinverted first such that the depth r with respect to the scan origingiven by (x₀, y₀) is related to signal time of the echo, r=√{square rootover ((x−x₀)+(y−y₀))}=c₀t/2, and i is the integer value ofi=[(tan((x−x₀)/(y−y₀))−θ₁)/Δθ]+1 where θ₁ is the angle of the firstA-line and Δθ is the angle between two consecutive A-lines. The envelopeof the rf-signal around A_(j)(t) is calculated to find signals aroundthe peak of A_(j)(t), until the envelope drops down to half of the peakamplitude. Echoes with these characteristics are identified as theechoes of secondary transmissions, that is, reverberation artifacts.

Let S_(ij)(t) indicate the received signals at the i'th receiver elementfollowing the j'th transmission (Tx_(j)). A_(j) is obtained bybeamforming the signals s_(ij) for all the receiving elements of thearray, as indicated by the multiline processors 110 a-110 n in FIG. 2.The beamforming delays used to construct A_(j) are used to identify thereceived signals such that if the signals from receiver i are delayed byϕ, and the echo time is calculated as t_(e), then the signals aroundt_(e)+ϕ are responsible for the reverb. For this purpose there is directcommunication between the unprocessed echo data from the probe and thereverb signal processor as shown in FIG. 2. The beamforming delays canbe read in real time from the multiline processors or alternatively thesame delays that are being used in the multiline processors can be alsopre-loaded to the reverb signal processor. From a detected envelope ofthe signals, the signals on both sides of the peak t_(e)+ϕ are examineduntil the envelope amplitude drops down to half of the maximumamplitude. This echo location and signal levels are saved for thereverberation removal processing. These signals are referred tohereafter as s^(rev)(t). The signals, s^(rev) _(ij)(t) from all thereceiving elements define the reverberation wavefront which is reflectedfrom the transducer surface following the j'th transmission. Thiswavefront is generally complex (non-planar, not converging, notuniformly apodized), because the anatomical surfaces they reflect fromare rarely flat.

Next, the reverberation signal processor performs a simulation of thereverb wave propagation, using the complex wavefront as input from thetransducer side of the signal paths. The simulated waves 22′ arepropagated outward from the transducer array 104 towards the focalpoints 130 of the beams as shown in FIGS. 4 a) and b) for a sectorgeometry to decompose the wavefront into a number of virtual pointsources. The amplitudes and arrival times of the propagated wavefront22″ are calculated for the focal points 130 of the beams as shown inFIG. 4 b). This is done by constructing a transmit-receive transformmatrix K_(TxRx)(t) using the signals s^(rev) _(ij)(t) for the i'thelement and j'th focused transmission:

${K_{TxRx}(t)} = \begin{bmatrix}{s_{11}^{rev}(t)} & {s_{12}^{rev}(t)} & K & {s_{1N}^{rev}(t)} \\{s_{21}^{rev}(t)} & {s_{22}^{rev}(t)} & \; & {s_{2N}^{rev}(t)} \\M & \; & O & M \\{s_{M\; 1}^{rev}(t)} & K & K & {s_{MN}^{rev}(t)}\end{bmatrix}$where M is the number of transducer elements and N is the number offocused transmissions. Each column of the K_(TxRx) matrix represents theper-element received reverberation signal data following a focusedtransmission. Similarly the focused transmit matrix K_(focus)(t) isconstructed as

${K_{focus}(t)} = \begin{bmatrix}{s_{11}(t)} & {s_{12}(t)} & K & {s_{1M}(t)} \\{s_{21}(t)} & {s_{22}(t)} & \; & {s_{2M}(t)} \\M & \; & O & M \\{s_{N1}(t)} & K & K & {s_{NM}(t)}\end{bmatrix}$where s_(ij)(t) denotes the signals being transmitted from each of Mtransducer elements for N focused transmissions. This computation isfacilitated by the data line between the transmit beamformer 106 and thereverb signal processor as shown in FIG. 2. This matrix includes thedelays and apodization weights associated with the transmit beamforming,which can be pre-loaded to the reverb signal processor. Both of thesetransformation matrices can also be represented in the frequency domain,K_(TxRx)(ω) and K_(focus)(ω), after Fourier transformation in the timedimension.

Any column of the matrix K_(TxRx) can be left multiplied with the matrixK_(focus) to simulate the propagation of the reverb signals to the focalpoints (virtual sources 130). Thus,V(ω)=K _(focus)(ω)K _(TxRx)(ω)where V(ω) is an N-by-N matrix whose columns represent the decompositionof reflecting reverberation echoes to the N virtual sources 130. Theresult is a complex (phase and amplitude) vector of N elements. Thiscalculation can be repeated for each frequency ω, typically by a Fouriertransformation of n different frequencies corresponding to the Nyquistrange of the signal sampling frequency, to decompose the reverberationwavefront from each transmit event into its virtual source components.An inverse Fourier transform of V(ω) is performed to get back to thearrival times and wave amplitudes at the focal points, which will serveas the delay times and weights for the virtual sources in the correctionperformed by the retrospective dynamic transmit focusing adjustmentdescribed above. FIG. 4 c) illustrates the virtual source points 130re-positioned as a function of delay time t_(delay), representing thedelay times needed at the virtual source points for correction of thereverberation artifacts.

The received A-lines from the individual transmit event Tx_(j), whichhave been stored in the line store 112, are combined (summed), afterapplying the standard retrospective dynamic transmit focusingcorrections, to estimate the A-lines for the secondary (reverberation)transmit, A^(rev) in the following equation:A ^(rev)(ω)=[V(ω)]^(H) A(ω)where superscript H indicates the Hermitian operator of a matrix. Inthis expression A(ω) indicates the column vector of an A line matrixexpression after Fourier transformation and A^(rev) indicates theapproximations to the received and beamformed A-lines if the secondarytransmit would have actually been transmitted. However, no physical beamtransmission is actually done and the signals responsible forreverberation artifacts are approximately calculated by the inverseFourier transform of A^(rew)(ω). Alternatively, per-channel data,instead of beamformed A-lines, from individual transmissions are usedfor retrospective dynamic transmit refocusing and are combined (summed)incorporating the weights and delays calculated to estimate the receivedsignals for the secondary (reverberation) transmit. Finally, theestimated reverberation echo signals of the virtual secondarytransmission are eliminated from the actual received signals bysubtraction:A ^(corrected)(ω)=A(ω)−A ^(rev)(ω)the corrected A-line is obtained by inverse Fourier transformingA^(corrected)(ω). Although the reverb cancellation process has beenillustrated in the frequency domain, it can also be performed in thetime domain. After the calculation of time delays and weights, thedelays can be applied to the beamformed A-lines using bitshifttechniques and weights can be multiplied with the delayed A-lines andthe summation can be carried out in the time domain to obtainA^(rev)(t). Finally, A^(rev)(t) is subtracted from A(t) to obtainA^(corrected)(t).

The foregoing processing and reverberation artifact correction isillustrated sequentially in the flowchart of FIG. 5. The block 70illustrates a method of creating an ultrasound image using the B modetransmit-receive sequence of a sequence of N transmit events (Tx #1, Tx#2, . . . Tx #N), the echo data from which is used to create theultrasound image. Each transmit event results in the reception of echosignals by each element (channel) in the transducer array, thisper-channel (per-element) data being shown as Rcv #1 . . . Rcv #N forthe N transmit events. The echo data from each transmit event isbeamformed into an A-line, thus forming A-L #1 . . . A-L #N. In step 72the set of echo data from Rcv #1 is analyzed to identify strong echoesresponsible for reverberation, which can include envelope-detection ofthe rf signals, and searching for amplitude peaks which are greater thana threshold value and a number (e.g., two) of standard deviationsgreater than those of surrounding pixels. In step 74 a simulation of thepropagation of an identified reverberation signal wavefront from thetransducer toward the virtual point sources 130 is performed using thesignal matrix equations given above, e.g., V(ω). An inverse Fouriertransform of V(ω) yields the arrival times and signal amplitudes at thefocal points which serve as the delay times and weights forreverberation correction as indicated in step 76. In step 78 the weightsw_(i) and delays t_(i) are applied to the beamformed A-lines A-L #1 . .. A-L #N from the N transmissions and in step 80 the weighted anddelayed A-lines are summed to estimate the received reverberationsignals (step 82). The method may further comprise a step 84 ofeliminating the reverberation signals from the A-lines. This process isrepeated for the Rcv #2 echo data set (step 88) to eliminate thereverberation signals from that set of data and after all data sets havebeen processed an image may be formed using the corrected A-lines (step86).

It should be noted that an ultrasound system suitable for use in animplementation of the present invention, and in particular the componentstructure of the ultrasound system of FIG. 2, may be implemented inhardware, software or a combination thereof. The various embodimentsand/or components of an ultrasound system, for example, the multilineprocessors, the weighting and delay circuits, the envelope detector andreverberation signal processor, and the image processor, and thecomponents and controllers therein, also may be implemented as part ofone or more computers or microprocessors. The computer or processor mayinclude a computing device, an input device, a display unit and aninterface, for example, for accessing the Internet. The computer orprocessor may include a microprocessor. The microprocessor may beconnected to a communication bus, for example, to access a PACS systemor the data network for importing training images. The computer orprocessor may also include a memory. The memory devices such as the linestore 112 may include Random Access Memory (RAM) and Read Only Memory(ROM). The computer or processor further may include a storage device,which may be a hard disk drive or a removable storage drive such as afloppy disk drive, optical disk drive, solid-state thumb drive, and thelike. The storage device may also be other similar means for loadingcomputer programs or other instructions into the computer or processor.

As used herein, the term “computer” or “module” or “processor” or“workstation” may include any processor-based or microprocessor-basedsystem including systems using microcontrollers, reduced instruction setcomputers (RISC), ASICs, logic circuits, and any other circuit orprocessor capable of executing the functions described herein. The aboveexamples are exemplary only, and are thus not intended to limit in anyway the definition and/or meaning of these terms.

The computer or processor executes a set of instructions that are storedin one or more storage elements, in order to process input data. Thestorage elements may also store data or other information as desired orneeded. The storage element may be in the form of an information sourceor a physical memory element within a processing machine.

The set of instructions of an ultrasound system including thosecontrolling the acquisition and processing of ultrasound images asdescribed above may include various commands that instruct a computer orprocessor as a processing machine to perform specific operations such asthe methods and processes of the various embodiments of the invention.The set of instructions may be in the form of a software program. Thesoftware may be in various forms such as system software or applicationsoftware and which may be embodied as a tangible and non-transitorycomputer readable medium. Further, the software may be in the form of acollection of separate programs or modules such as ones executing thesimulation and processing of the equations of the reverberation signalprocessor described above. The software also may include modularprogramming in the form of object-oriented programming. The processingof input data by the processing machine may be in response to operatorcommands, or in response to results of previous processing, or inresponse to a request made by another processing machine.

Furthermore, the limitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. 112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function devoid of further structure.

The invention claimed is:
 1. A method for reducing reverberation signalartifacts in ultrasound images comprising: receiving ultrasound echosignals along a plurality of multiline A-lines received in response to acommon transmit event which contain reverberation signal artifacts;processing the ultrasound echo signals by envelope detection; detectingreverberation signal components in the ultrasound echo signals from anamplitude of envelope detected ultrasound signals in relation to areference; estimating time delays and amplitudes for reverberationsignal correction from a simulation of transmission of reverberationsignals to virtual point sources; combining a plurality of the A-linesof the ultrasound echo signals using the estimated time delays andamplitudes to reduce reverberation signal components in the combinedA-lines; and forming an ultrasound image using the combined A-lines. 2.The method of claim 1, wherein receiving ultrasound echo signals furthercomprises transmitting a plurality of transmit events and receiving aplurality of multilines in response to each transmit beam.
 3. The methodof claim 2, wherein combining a plurality of multilines furthercomprises combining a plurality of multilines using time delays whichrefocus the multilines with respect to each other.
 4. The method ofclaim 1, wherein detecting reverberation signals further comprisesidentifying ultrasound signal pixel amplitudes which are larger than acertain threshold and at least two standard deviations greater thansurrounding pixels.
 5. The method of claim 1, wherein the virtual pointsources are located at focal depths of transmitted or receivedultrasound beams.
 6. The method of claim 1, wherein estimating timedelays and amplitudes for reverberations signal correction furthercomprises estimating reverberation signal components from the receivedecho signals.
 7. The method of claim 6, wherein the virtual pointsources are located at focal depths of transmitted or receivedultrasound beams; and wherein the time delays and amplitudes areestimates from arrival times and amplitudes of simulated reverberationsignal components at the virtual point sources.
 8. The method of claim6, wherein combining a plurality of A-lines of the ultrasound echosignals using the estimated time delays and amplitudes to reducereverberation signal components further comprises subtracting estimatedreverberation signal components from the received ultrasound echosignals.
 9. The method of claim 7, wherein combining a plurality ofA-lines further comprises combining a plurality of A-lines usingrefocusing delays to estimate A-lines containing reverberationartifacts.
 10. A diagnostic ultrasound system for reducing reverberationsignal artifacts in ultrasound images comprising: multiline processorsarranged to provide a plurality of A-lines by at least partiallybeamforming a plurality of received ultrasound echo signals whichcontain reverberation signal artifacts; an envelope detector responsiveto the plurality of A-lines and arranged to detect an envelope of eachA-line, wherein each envelope detection includes an envelope amplitudedetection; a reverberation signal processor coupled to the envelopedetector and arranged to detect reverberation signal components in theultrasound echo signals from an amplitude of the envelopes, and toestimate time delays and amplitudes for reverberation signal correctionfrom a simulation of transmission of reverberation signals to virtualpoint sources; a combiner coupled to both the multiline processors andthe reverberation signal processor and arranged to combine the pluralityof A-lines of the ultrasound echo signals using the estimated timedelays and amplitudes to reduce reverberation signal components in thecombined A-lines; and an image processor arranged to form an ultrasoundimage using the combined A-lines.
 11. The diagnostic ultrasound systemof claim 10, wherein the reverberation signal processor is arranged toestimate time delays and amplitudes for reverberation signal correctionby further calculating a product of a focused transmit signal matrix anda transmit-receive transformation matrix.
 12. The diagnostic ultrasoundsystem of claim 11, wherein the reverberation signal processor isarranged to estimate time delays and amplitudes for reverberation signalcorrection by further calculating an inverse Fourier transform of acomplex reverberation wavefront matrix.
 13. The diagnostic ultrasoundsystem of claim 10, wherein the image processor is arranged to form anultrasound image by forming an image from a plurality of multilineswhich have been refocused as a function of axes of their transmit beams.